We propose to study the binding of nucleotides and aminoacyl-tRNAs (AA-tRNAs) to elongation factor Tu(Ef-Tu), the small bacterial protein (MW 42,000) responsible for carrying AA-tRNAs to the ribosome during protein biosynthesis. The initial studies are directed toward estimating the spatial relationships of the nucleotide and AA-tRNA binding sites with respect to the sulfhydryl group (SH2) of EF-Tu required for tight AA-tRNA binding, using magnetic resonance techniques. The methods developed here will be applied in later work to define aspects of specificity in Ef-Tu-nucleic acid interactions. Distances on the protein will be estimated using the distance-dependent dipolar relaxation (T1p or T2p) of an observed diamagnetic nucleus by a covalently bound paramagnetic nitroxide spin label in the complexes Ef-Tu.MgGDP and Ef-Tu.MgGTP.AA.tRNA or their equivalents. The method requires a knowledge of the fraction of fast exchange prevail between the observed nucleus in diamagnetic and paramagnetic environments. Considerable effort will also be directed toward the necessary prerequisities of (1) devising methods to obtain fast exchange of nucleotide and AA-tRNA from Ef-Tu complexes, (2) observing nucleotide resonances in diamagnetic Ef-Tu complexes and (3) obtaining appropriate dissociation constants.